1. Field of the Invention
The invention relates to the manufacture of pistons.
2. Review of the Prior Art
It has been customary to manufacture the basic piston shape by a single piston-forming process; by forging, by gravity die casting or, more recently, by squeeze casting methods in which molten metal is gravity fed to an open die, the die then being closed and the metal solidified under a force of many tonnes. Each of these methods has advantages in the formation of certain parts of the piston but disadvantges in the formation of certain other parts of the piston.
In the case of forging, this method has the benefit of producing high strength so that highly stressed parts of the piston, such as gudgeon pin bosses, are very strong when forged. It has the disadvantage, however, that it is difficult to produce crown end piston features, such as combustion bowls or piston ring grooves, by forging; these require subsequent machining and, possibly, shaping operations.
Gravity die casting has the advantage that pistons can be produced easily and cheaply by this method. The incorporation of inserts can be readily accomplished by using, for example, the Al-Fin bond. Gravity die casting has the disadvantage, however, that it does not have a high strength, as compared with forging, and is not particularly resistant to the elevated temperatures often found at the crown end of a piston, particularly where that piston is for a diesel engine.
Squeeze casting has the advantage that a piston so produced has a more uniform structure than gravity cast pistons, with an absence of voids. This makes the piston mechanically stronger as well as improving generally the properties of material. The production of pistons by squeeze casting is, however, more expensive and time-consuming than their production by conventional gravity die casting. In addition, while the improved properties are desirable or necessary in certain parts of the pistons, such as the crown region, they are not as desirable or necessary in other portions of the pistons, such as the lower skirt region. This is particularly true where the pistons are very large diesel pistons where the crown is subject to high temperatures and where the volume of material involved in the manufacture of the piston makes squeeze casting expensive.